1. Field of the Invention
The present invention relates to an optical apparatus which uses as a light source a semiconductor laser which has different focal points in a junction plane and in a plane perpendicular thereto to cause an astigmatism and, more particularly, to an optical reader suitably adapted for an audio or video disc player, which focuses light rays from a semiconductor laser onto a reading surface of an optical disc so as to read signals recorded on the reading surface.
2. Description of the Prior Art
A semiconductor laser of the gain guiding type as one type of double heterojunction semiconductor laser is free from an increase in noise level which is frequently encountered in a semiconductor laser of the index guiding type. Such an increase in noise level is caused by a self-coupling effect which is, in turn, caused by the light reflected from the reading surface of the optical disc. For this reason, a semiconductor laser of the gain guiding type is considered as a promising light source for an optical reader of a video disc player which requires a high S/N ratio. A semiconductor laser of the gain guiding type oscillates vertically in multimode oscillation, while a semiconductor of the index guiding type oscillates vertically in a single mode. Accordingly, the semiconductor laser of the gain guiding type is less subject to interference from reflected light. However, from the viewpoint of optical characteristics, as shown in FIGS. 1(A) and 1(B), oscillated light rays from a semiconductor 1 of the gain guiding type have different mode waists in the junction plane (X-Y axis plane) and in a plane perpendicular thereto (X-Z axis plane). More specifically, in the vertical plane (X-Z axis plane), the mode waist is at point A (mirror surface position) which lies in the plane of a mirror surface 2. However, in the junction plane (X-Y axis plane), the mode waist is at point B which corresponds to an active layer 3 of the semiconductor laser, and lies at a point deeper inside the resonator from the mirror surface 2. For this reason, the focal points of the oscillated light rays in the junction plane (X-Y) plane and the vertical plane (X-Z) plane differ from each other to cause an astigmatism .DELTA.Z.
When a semiconductor laser of this type is used as a light source for a video disc player or the like, and light rays therefrom are focused by an objective lens or the like onto the reading surface of an optical disc, the spot is distorted into a horizontally or vertically elongated shape due to the astigmatism. When this occurs, a point for obtaining optimal reproduction data signals and tracking error signals cannot be determined. This reduces the margin for disturbance against servo such as defocusing or disc skew. In other words, desired OTF (Optical Transfer Function) characteristics of the optical system may not be obtained.
In view of this problem, the following methods have been conventionally used:
(a) According to the first method, those components of the light rays diverging from the semiconductor layer which are within a narrow central angle range are selected to be used for reading signals so as to eliminate disturbance in the wave front due to the astigmatism. The degree of adverse effect due to the astigmatism varies according to the NA (Numerical Aperture) of a collimator lens which is used to guide the light rays onto the objective lens. Therefore, if only those light components which are within a narrow central angle range are selected for reading signals, disturbance in the wave front is eliminated, although the efficiency of use of the light rays is degraded. Thus, desired OTF characteristics may be obtained in the case of a digital audio disc (DAD) which does not require too high an S/N ratio.
This will be explained in further detail. Since the S/N ratio required is not too high, a DAD player does not require too high an optical density. For this reason, a collimator lens having an NA of 0.13, for example, may be used. Then, even if a semiconductor laser having an astigmatism of 25 .mu.m is used as a light source, the RMS value of the disturbance in the wave front is 0.056 (.lambda.), which is within the diffraction limit, thus providing no problem.
However, when the first method described above is applied to a video disc player or the like which requires a relatively high S/N ratio, the laser output must be increased due to the low efficiency of light rays. Such an increase in the laser output gives rise to a problem of short service life of the semiconductor laser.
If a currently available semiconductor laser of this type is to be used as a light source for a video disc player or the like which requires a high S/N ratio without adopting the first method, then a collimator lens having an NA of 0.2 or more must be used in consideration of the angle of divergence of the light rays from the semiconductor laser. However, if such a collimator lens is used, the RMS value of the disturbance in the wave front due to an astigmatism of 25 .mu.m becomes 0.13 (.lambda.), which significantly degrades the OTF characteristics.
The RMS value of the disturbance in the wave front which is within the diffraction limit is known to be 0.07 (.lambda.) (Marechal Criterion). The upper limit of astigmatism of a laser which satisfies such a criterion must be 13 .mu.m if the NA of the collimator lens is assumed to be 0.2. The astigmatism of a currently available semiconductor laser of the gain guiding type is about 20 to 25 .mu.m. For this reason, if a light source of a great light intensity is required, as in the case of a video disc player or the like, some measures for correcting the astigmatism must be taken.
(b) According to the second method, the astigmatism is corrected by an optical element such as a cylindrical lens which has different power (diffraction capacity) in different directions.
However, when the second method is adopted, since the power of the optical element is different in different directions, the optical element surface, that is, the lens surface, does not become a true sphere but an irregular sphere. Such an irregular sphere is difficult to design and manufacture. Furthermore, since the power of the optical element is different in different directions, various positioning adjustments of the optical element must be made including the angular position of the optical element with respect to the optical axis, the position of the element along the optical axis, and the directivity of the power in relation to the astigmatism. This complicates the positioning procedures of the optical element.